Name two processes that are important in the carbon cycle. Options: a. Photosynthesis and nitrification b. Nitrification and respiration
c. Photosynthesis and respiration d. Nitrification and denitrification

I'm thinking it's A but am unsure.

Which of the following must be true for a fossil organism to be useful as an index fossil? a. the fossil organism must be widespread geographically. b. the fossil organism must be abundant. c. the fossil organism must be limited to a short span of geologic time. d. all of the above.

What are the subsystem of the earth

What does spontaneous generation mean?